1. Technical Field
The present invention relates generally to semiconductor piezoresistive devices. More particularly, the invention relates to a method for production of semiconductor piezoresistive devices wherein a thin film of crystalline silicon is formed on a substrate substantially in accordance with the plasma chemical vapor deposition (plasma CVD) method.
2. Background Art
A variety of methods for production of semiconductor piezoresistive devices have been proposed so far. For example, these devices can be obtained by cutting a single crystal into rectangles or from a thin film formed by the physical vapor deposition (PVD) method such as vacuum deposition. The piezoresistive device thus obtained is stuck, adhesive-bonded or vacuum-evaporated as a pressure sensitive member onto a strain-receiving member (e.g., metal sheet, bellows or metal diaphragm) and the like, and variations in the specific resistance (piezoresistance effect) of the device due to variations in the strain of the strained member is utilized. The device is applied to pressure sensors and the like.
In another method, the semiconductor piezoresistive devices can be produced by forming in a single crystal substrate a diffusion layer different in type from the substrate. In the case of the single crystal substrate thus produced, the semiconductor substrate acts per se as a slrain-receiving member, and the diffusion layer in the semiconductor forms a piezoresistive device.
The former two methods which involve cutting a single crystal and vacuum evaporation respectively can provide piezoresistive devices having the advantageous features of low temperature coefficients with respect to the resistivity and gauge factor coupled with a wide temperature range in which the devices are operable, but cannot be employed to form such devices into IC's having an active function.
The latter diffusion method, on the other hand, can ensure mass production of piezoresistive devices and formation of IC's therefrom. In the piezoresistive device obtained by this method, however, a saturation current which is dependent on temperature flows into the p-n junction, where the substrate and the device are separated from each other (reversely biased), whereby the temperature range in which the device is operable is limited. Further, the piezoresistive device produced by the diffusion method has high temperature coefficients with respect to both the resistivity and the gauge factor, i.e., 2000 ppm/.degree.C. and -1000 ppm/.degree.C., respectively. For this reason, temperature compensation by either active devices such as transistors formed discretely or simultaneously or thermo-sensitive resistors is required. This results in reduced reliability and lowered responsiveness due to complicated process steps and hence an increased number of devices